MORPHOLOGY OF ANIMALS 



end to form the brain (Fig. 4, E, n). This neural tube, 

 while apparently a continuous structure, is really com- 

 posed of segments, the neuromeres^ one neuromere being 

 formed in each body somite ; the neuromeres are thus com- 

 parable to the ganglia of the ventral chain of arthropods 

 and annelids. This segmentation of the central nervous 

 system of vertebrates, is indicated even in the adult by the 

 segmental arrangement of the spinal and cranial nerves. 

 In the embryonic development of all vertebrates the brain 

 consists of three enlargements or vesicles, the fore-brain^ 

 mid-brain^ and hind-brain (Fig. 24, A); the first gives 

 rise to the cerebrum and 'tween brain of the adult, the 

 second remains as the mid-brain^ while the third gives rise 

 to the cerebellum and medulla (Fig. 24, C). The portion 

 of the neural tube posterior to the brain becomes the 

 spinal cord of the adult. With the differentiation of nerve 

 cells and fibres in the walls of the neural tube these walls 

 increase greatly in thickness, while the originally large 

 cavity of the tube becomes restricted in size, forming in 

 the adult the ventricles of the brain and the central canal 

 of the cord (Fig. 24, K). 



10. Sense Organs 



The simplest sense organs are the scattered sensory cells 

 found in the superficial epithelium of many animals ; these 

 may be solitary or aggregated into buds. They are elon- 

 gated epithelial cells with a hair-like process at the free 

 border and a fibre at the deeper end connecting with the 

 branches of a nerve cell (Fig. 23, B). They are organs of 

 general sensation, — that is, they are capable of receiving 



